Plastics, particularly polycarbonate (PC) substrates, provide unique properties such as transparency, lightweightedness, handleability, and ease of formability. These properties make them especially useful as substitutes for glass in many applications. However, plastic substrates do have their shortcomings in that typically they are soft and scratch readily. Thus, several siloxane-based curable resin systems have been developed to provide exceptionally hard, abrasion resistant protection for plastics approaching that of glass itself. These siloxane-based coatings are considered to be the premier coatings in the plastic sheet and lens market today.
A number of improvements in these silane-based compositions have been developed in the last 30 years that have resulted in improved coatings for plastics in a number of aspects. For instance, in the late '70s, coating compositions containing at least one hydrolyzate of an epoxysilane, a polycarboxylic acid or anhydride crosslinker, and a curing agent for the epoxy functionality were developed. Although these coatings provided protection to plastic surfaces, such as polycarbonates, they were not particularly abrasion resistant compositions.
In the early '80s, siloxane formulations comprising at least two hydrolyzates of: 1) an epoxy silane, and 2) a trifunctional silane, predominately methyltrimetboxysilane (MTES); aqueous colloidal silica; the aforementioned crosslinkers and curing agents were developed. These compositions did not prove to be particularly abrasion resistant due to suppression of crosslinking by the blocking methyl group in the trifunctional silane, MTES.
In the '90s, efforts were successful in increasing the silane network crosslinking (and thereby increasing the abrasion resistance) by replacing the trifunctional silane (MTES) with the tetrafunctional silane, e.g., tetraethoxysilane (TEOS). Most of the other components of earlier formulations were retained, namely, the epoxy silane, the acid crosslinker, the curing agent (where needed), and optionally, the aqueous basic colloidal silica sol (Na+ stabilized sol). The Na+ stabilized sol with pH in the 9 to 10 range presents some issues in the mixing process, because the final pH of the composition is acidic, typically between about 4 and about 5.5. If the silica sol is not properly added, it can lead to flocculation, as the sol must transverse the neutral pH range where silica is very unstable. This can force a change in the order of addition of the silica sol in certain compositions. Also, residual alkali may be present in the coatings derived from these compositions, which may not be desirable in certain applications.
Many of the aforementioned coating compositions do not provide adequate protection under a variety of conditions. Maintenance of internal cohesive strength has been a problem for many plastic substrates coated with abrasion resistant coatings, particularly under extreme climatic conditions. Cohesive failure results in cracking and peeling of the abrasion resistant coating, and thereby exposing the underlying substrate to abrasion, pitting, and physical or chemical attack.
Another existing problem relates to particular commercial applications of coated polymeric substrates, which applications require the coatings to withstand high temperature and high humidity for long time periods (years) (e.g., auto windows). When tested for this requirement, via accelerated tests such as exposure to boiling water, most of the aforementioned abrasion resistant coatings fail in less than 1 hour due to a crazing mechanism.